1. Field of the Invention
The present invention relates to photovoltaic devices, and more particularly, to improving the photoelectric conversion efficiency of a photovoltaic device including a thin film of an amorphous semiconductor.
2. Description of the Background Art
Reduction in cost and improvement in photoelectric conversion efficiency are desired characteristics for photovoltaic devices. A photovoltaic device including an amorphous semiconductor film can be formed in a thin configuration because light absorption thereof is higher than that of a photovoltaic device including a crystalline semiconductor layer. The amorphous photovoltaic device has an advantage that the manufacturing process is simple. There is also an advantage that energy can be saved at the time of manufacturing due to a low temperature process. The amorphous photovoltaic device can use economic glass, stainless steel, ceramic or the like for the substrate material to reduce the cost.
However, when glass, stainless steel, ceramic, etc. are used as the substrate material, the light reflectance of the surface of these materials is generally low. Therefore, the reflectance of the substrate surface is increased by forming a smooth reflecting layer on the substrate using one or more kinds of metal having high reflectance to improve the photoelectric conversion efficiency. A method is taken to improve the photoelectric conversion efficiency by reflecting the incident light at the substrate surface not absorbed by the semiconductor film of the photovoltaic device and making this reflected light be absorbed by the semiconductor film.
However, interference occurs between the incident light and the reflected light to produce interference fringes when light passing the semiconductor film is mirror-reflected from the substrate surface and is again introduced into the semiconductor film. Interference fringe is a phenomenon where light waves weaken or strengthen each other. This means that the intensity of the incident light and the reflected light interact each other to be weakened, depending on the thickness of the semiconductor film and the wavelength of the incident light. This is relatively equivalent to a reduction in the luminance energy, which results in a reduction of the photoelectric conversion efficiency.
Referring to FIG. 15, a schematic sectional view of a known photovoltaic device is shown that is expected to improve photoelectric conversion efficiency by reducing the above-mentioned light interference. This photovoltaic device is provided with a reflective metal layer 2 having an uneven surface on a substrate 1. The reflective metal layer 2 serves as a back electrode. An amorphous semiconductor film 3 serving as a photoelectric conversion layer is formed on the back electrode 2. The semiconductor film 3 is covered by a transparent front electrode 4.
An incident light beam L.sub.0 into such a photovoltaic device that is not absorbed within the semiconductor film 3 reaches the back metal electrode 2, whereby the incident light beam L.sub.0 is scatter-reflected on the uneven surface of the back metal electrode 2. If the scatter-reflected light L.sub.1 is not absorbed within the semiconductor film 3 and returns to the interface of the semiconductor film 3 and the transparent front electrode 4, light L.sub.1 is likely to become a light L.sub.2 reflected again at the interference back into the semiconductor film 3, due to the tendency of L.sub.1 establishing a relatively large angle with respect to the normal line of the aforementioned interface. In other words, incident light once entering the semiconductor film 3 is apt to be confined within the semiconductor film 3. This is called "light confinement technology".
As a result, interference between light reflected from the back metal electrode 2 and the incident light is reduced in the photovoltaic device of FIG. 15. The scatter-reflected light from the back metal electrode 2 is efficiently absorbed within the semiconductor film 3 because the optical path length of the reflected light becomes longer and contributes to photoelectric conversion.
One method of forming a back metal electrode having an uneven surface includes the steps of depositing a silver film having a thickness of approximately 1000.ANG. at approximately 260.degree. C. on a substrate by electron beam evaporation, and baking in vacuum the same for obtaining an uneven surface of the silver film. Another conventional method includes the steps of depositing a silver film of approximately 1000.ANG. in thickness at approximately 200.degree. C. on a substrate, and baking the same at approximately 600.degree. C. for obtaining an uneven surface of the silver film.
It is however difficult to form a satisfactory uneven surface on the back metal electrode suitable for "light confinement" using the method of baking. Furthermore, such baking at a high temperature of 600.degree. C. may exert a bad influence on the substrate.